Capacitive and inductive coupling between transmission lines may lead to electromagnetic signals propagating in one transmission line adversely affecting electromagnetic signals propagating in another transmission line. This is often referred to as crosstalk. Transmission lines are common in computer systems, where a chip (die) may communicate with another chip via transmission lines. For example, part of a computer system is shown in FIG. 1, where CPU (Central Processing Unit) 102 communicates with chipset 104 via front side bus 106. Chipset 104 comprises a memory controller to communicate with memory via memory bus 110, and has a port for communicating with graphics processor 112 via graphics bus 114. Chipset 104 also comprises a bridge for communicating to other peripherals, such as network interface controller 116, via system bus 118. The busses in FIG. 1 may considered transmission lines.
The crosstalk problem is illustrated in FIG. 2, where a pulse representing a logical “1” is transmitted by driver 202 on transmission line 204, and driver 206 indicates a logical “0” by not transmitting a signal on transmission line 208. (For simplicity, only one conductor for each transmission line is shown, but in practice there may be one or more planar conductors forming part of the transmission line structure.) The received pulses are depicted in FIG. 2, where pulse 210 depicts the received pulse on transmission line 204, pulse 212 depicts the received pulse on transmission line 208, and where t1, t2, and t3 are three instances of time for which t1 is the rise of pulse 210, t2 is the midpoint of pulse 210, and t3 is the fall of pulse 210. Pulse 212 results from crosstalk, and is approximately the derivative of pulse 210.
If pulses 210 and 212 are aligned as shown in FIG. 2, and sampling occurs at t2, then received pulse 212 on transmission line 208 is correctly interpreted as a logical “0”. However, there may be timing jitter so that received pulse 212 may not be sampled at time t2, or there may be electrical length mismatch among transmissions lines 204 and 208 so that the received pulses are not aligned as indicated in FIG. 2. Consequently, pulse 212 may be sampled so that it is interpreted incorrectly as a logical “1”.
A transmission line may take various physical forms, such as a stripline or a microstrip. A stripline may comprise a trace between two planar conductors, whereas a microstrip may comprise a trace adjacent to one planar conductor. Crosstalk may be less of a problem for a stripline than a microstrip. However, striplines are more costly than microstrips. Consequently, the problem of crosstalk may be a common problem in computer systems, especially as signaling rate increases.